db: Filter out all generated foreign key constraints

While testing it was found that if all foreign key constraints were
removed from the generated schema migrations, total chain sync time
on mainnet dropped from 6 days to a little over 3 days.

These constraints do very little to improve data integrity of the data
in the database because they are already checked in Haskell land. They
can therefore be dropped.

The Haskell land code will however need to be modified/updated to work
around the lack of these constraints. For example removing these
foreign key constraints means that `OnDeleteCascade` will no longer
work and will therefore have to be done manually. This is a small price
to pay for the sync speed improvements.

cardano-db/src/Cardano/Db/Migration.hs

@@ -169,7 +169,7 @@ createMigration source (MigrationDir migdir) = do
     create :: ReaderT SqlBackend (NoLoggingT IO) (Maybe (MigrationVersion, Text))
     create = do
       ver <- getSchemaVersion
-      statements <- getMigration migrateCardanoDb
+      statements <- filterMigrations <$> getMigration migrateCardanoDb
       if null statements
         then pure Nothing
         else do
@@ -212,6 +212,14 @@ createMigration source (MigrationDir migdir) = do
           let (SchemaVersion _ stage2 _) = entityVal x
           pure $ MigrationVersion 2 stage2 0
 
+filterMigrations :: [Text] -> [Text]
+filterMigrations =
+    filter keepMigration
+  where
+    keepMigration :: Text -> Bool
+    keepMigration txt =
+      not (Text.isInfixOf "ADD CONSTRAINT" txt && Text.isInfixOf "FOREIGN KEY" txt && Text.isInfixOf "REFERENCES" txt)
+
 recreateDB ::  PGPassSource -> IO ()
 recreateDB pgpass = do
     runWithConnectionNoLogging pgpass $ do